docs: def to_datetime_field(formatter):
      """
      Returns a callable instance that will convert a string to a DateTime.
  
      :param formatter: String that represents data format for parsing.
      :return: instance of the DateTimeConverter.
      """
      class DateTimeConverter(object):
  
          def __init__(self, formatter):
              self.formatter = formatter
  
          def __call__(self, value):
              if isinstance(value, string_types):
                  value = parser.parse(value)
  
              return value
  
      return DateTimeConverter(formatter)